You need to create a new Policy for them for use in the Context Access ACL entry for the 'mgr' context with any permissions you don't want them to have unchecked in the Policy.
BTW, in Revolution, there are no 'web' and 'manager' users, there are just users. Context Access ACL entries for User Groups control what they can do and see in the 'web' and 'mgr' contexts.
You might find this helpful:
http://bobsguides.com/revolution-permissions.html
You may find that the users need to have too much power to do what you want to have them do. Revolution is not very good about letting users manage a subset of other users. If so, you may have to create a custom snippet or a Custom Manager Page (CMP) that lets them create and manage users after logging in.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html